Re-using yeast is a wonderful skill for any homebrewer to learn. It’s a milestone in your development as a brewer when you start to cultivate and reuse yeast not just because of the technical skill required, but it also means that more tasty beer is ahead!

Working with live yeast is not the easiest thing in the world to do, but it’s also not that hard. In today’s post, I’ll show you a few quick and relatively easy steps that will have you reusing your yeast in no time!

First Things First

Before we dive right in, let’s first go over some basics. Working with yeast presents a lot of risks for potential contamination of your live yeast. So the absolute best advice I can give you before we begin is: Keep things sterile. I mean it. REALLY Sterile.

However, there is one catch: Yeast reacts poorly to most sterilizing agent (e.g. Periscetic acid) and for good reason. These sterilizers are designed to kill close cousins of beer yeast: wild yeast and other fungi. So, be cautious in your use of sterilizers.

Spray rather than soak. A better option is to use a “clean” form of sterilization: heat. Boiling your equipment and/or exposing it to flame is a safer option for working with yeast.

To properly harvest, wash and reuse yeast, you’ll need a bit of kit. So let’s take a look at the gear you’ll need next..

Equipment

You probably already have most of the equipment needed for yeast harvesting and washing. Here’s what you’ll need to get:

    • 3 or 4 large mason jars with metal lids (of at least 500ml or more capacity)
    • A large (1-2 liter) sealable bucket or glass jar
    • Plenty of Brew Water (more on this later)
    • Sterilizer in a spray bottle
    • A large pot that can be used for boiling your mason jars in

Step 1: Harvesting Yeast

To reuse yeast, first you must harvest it. And harvesting yeast from a fermented beer is an important first step to get right. Here are a few pointers:

  • Harvest from lower alcohol beers, like Blonde Ales or Brown Ales. High alcohol is a known yeast stressor, and you want your harvested yeast cells to be as viable as possible
  • Don’t wait too long! After cold crashing (if you have a conical fermenter) or directly after bottling (if you don’t), you should harvest your yeast. Leaving it on fermented beer for too long is not the way to go.
  • When harvesting, note the color and aroma of the yeast cake you find at the bottom of the fermenter. Fresh, healthy yeast is a rich cream color and smells “bready”. Grey colours and funky aromas are not good. Chuck that stuff and look for when the yeast runoff becomes rich and creamy

The basic method of harvesting yeast is pretty simple. It is always better to work with chilled beer, so keep that in mind. But if you cold condition your beers, that shouldn’t be a problem. If not, let the fermentation bucket sit in the fridge for a day before you do your harvesting.

Next, decant the leftover yeast and other stuff into your large glass flask or bucket. Try to get as little as possible of the very bottom, dark stuff (known as trub) into your initial harvesting vessel. So, once the flow turns from a creamy color to dark grey or green, stop pouring!

Well done. You’re almost there!

Step 2: Washing your Yeast

 Once you’ve collected the leftover yeast (and trub) in your vessel, place it in the fridge for at least four hours. This will allow the different layers within your solution to settle. Throughout this process, you’ll be working with three different layers: 

  1. The Bottom: The trub. Dark grey or green in color, these are the remains of your wort, and includes dead yeast cells, spent grains, settled proteins and hop residue. Generally, you want little to none of this stuff in the final liquid yeast you’ll be storing and reusing.
  2. The Middle: The yeast. This is where you should have healthy, creamy yeast. Good healthy yeast is cream-colored and smells like bread. It should have no serious off-flavors or aromas. It’s this middle layer that we want to harvest and reuse.
  3. The Top: This thin layer may be transparent, beer-colored or somewhat grey. It contains more dead yeast cells, water, beer and alcohol. You also want as little as possible of this layer to remain in your final product, although it will be generated in stored yeast naturally and may need to be decanted from time to time, depending on how long you plan on storing your liquid yeast.

Now that your leftovers are nicely chilled, you’ll be washing the yeast. This will allow for the layers to settle more substantially and make it easier for you to pour off the stuff you don’t want (i.e. beer and trub) from the stuff you do want (healthy yeast).

This is where having plenty of sterilized brew water is a good thing. For washing yeast, you’ll need to go an extra few steps to sterilizing your water than what is the case when brewing beer. Remember that in brewing, you boil your wort for at least an hour. So too with water used to wash yeast, although 10-15 minutes of boiling is sufficient. Here are the steps:

  • Boil enough brew water (I prefer bottled mineral water for this) to fill your mason jars (boil time: 10-15 minutes)
  • In a separate container, boil your mason jars and lids for 10-15 minutes to sterilize them
  • Now, chill your brew water to match the temperature of the leftover yeast (i.e. around 4 degrees Celsius)
  • Once the water is chilled, fill about half of the mason jar with leftover yeast and top it up with your chilled brew water
  • Continue doing this until all your mason jars are filled or you’ve run out of leftover yeast. Do not be greedy! Once the leftovers reach the dark trub, stop pouring!
  • Now, seal your mason jars and give each of them a good swirl for a minute or two to mix the water and leftover yeast. You don’t have to shake them. In fact, be gentle and just make sure it’s all mixed in thoroughly
  • Place the mason jars back in the fridge and leave them there for at least 40 minutes (although 2-3 hours is even better).

What you’ll see once you take the jars out of the fridge is a clearly differentiated series of layers (see above). The cold temperatures and the water wash would have separated your healthy yeast from the beer and trub. The top layer will now contain mostly old yeast cells, alcohol and beer. There should be no bottom layer of trub any more, but if there is, no worries. We will be washing our yeast more than once anyway.

Now, pour out the top leftovers from each mason jar and stop pouring when you reach the healthy creamy yeast. Add more chilled water, swirl, and place back in the fridge. After 2-3 repeats, you should be left with a mason jar with only healthy yeast remaining

Congratulations: you have just successfully washed yeast! 

Step 3: Storing your Yeast

Once you’ve washed your yeast, you can store the mason jars in the fridge for around 2 weeks. I’d recommend against storing for longer, because liquid yeast viability declines very sharply after this period. Commercially purchased liquid yeast is more stable, but for home-made liquid yeast, 2 weeks is about the limit

Once you’re ready to reuse the yeast, you’ll need to make a yeast starter to get the most out of your freshly harvested yeast

Step 4: Reusing your Yeast

To re-pitch your harvested and washed yeast, you’ll be making a yeast starter from it a day before you plan to brew your next beer

To fully understand yeast pitching rates is a very brain-burning topic and I won’t go into major details here. Suffice it to say that your harvested yeast will not have as large a cell count of viable yeast cells as commercially available ones. Of course, you can take your harvested yeast to a lab and get a proper cell count, but barring that, it’s a safe assumption that one ml of yeast slurry (i.e. the stuff in your mason jars) contains about 1 Billion viable yeast cells.

Now that you know that, you can plug in the numbers into one of many pitching rate calculators online (or in packages like BeerSmith) to see how big a starter you’ll need.

As an example, if you’re making an ale with an OG of 1.045, you’ll need to pitch around 159 Billion yeast cells (although it’s often a good idea to overestimate this number by around 50-100 Billion). To get there, you’ll prepare a starter, which is a “mini-beer” that is used to get your liquid yeast started (hence the name) and grow a few additional healthy yeast cells in the process

Of course, you could just pitch your liquid yeast without making a starter, but because the yeast will be in a suspended state (i.e. at fridge temperatures), it needs to be revived, fed and brought up to pitching temperature (20-25 degrees C). A yeast starter helps with all of that.

Here’s what to do:

  • For the example above, you can start with 100ml of yeast slurry and make a 2 liter starter. That will ensure a good quantity of yeast cells in the final pitching. If you add more slurry, you need a smaller starter, and if you use less, you’ll be making a bigger starter.
  • Measure out your yeast slurry into a covered sterilized jar and let it come to room temperature
  • To make the starter, use 2 liters of brew water and 200 grams of Dried Malt Extract. This will produce a starter of about 1.040 gravity points, which is a good balance of nutrients and sugars for your yeast to eat. Mix the DME and water thoroughly and then boil it for 15 minutes.
  • Chill the wort until it’s at the same temperature as the yeast slurry and then pitch the slurry into your wort
  • For this, you can use a small bucket, a glass flask or anything that will accommodate the wort plus yeast combo. Seal by placing some sterilized foil over the mouth of the bottle or flask. You could use an airlock, but this isn’t really necessary since you’ll only have the starter ferment for about 12-24 hours before use

On brew day, you’ll have a nice, bubbly batch of liquid yeast to use in your new beer. Pour all two liters into your chilled wort as normal and away you go!

Final Thoughts

Harvesting and reusing yeast can be a complicated production but is well worth trying. Not only are you going to be saving money, but you’ll notice an improvement of flavor in your beer with each subsequent generation of yeast.

Note however that at most, you can re-use the same yeast colony about 8-10 times before it will no longer be that viable. Some ale yeasts can be reused way more than this, but for the DIY yeast harvester, 8-10 generations are more realistic.

As long as you stay sterile and use each new slurry of yeast within 2 weeks, you should be just fine. Happy harvesting!

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